Oxidation ovens are commonly used to produce carbon fibers from a precursor (such as an acrylic, pitch, or cellulose fibers). One common processing method involves successively drawing fibrous segments of the precursor material through one or more oxidation ovens.
Each of the oxidation ovens comprises a respective oxidation chamber in which the oxidation of the fiber segments takes place. Each fibrous segment can be drawn into a first oxidation oven as a carbon fiber precursor and then make multiple passes through each oxidation oven prior to exiting the final oxidation oven as an oxidized fiber segment. Roll stands and tensioners are used to draw the fibrous segments through the oxidation chambers of the ovens. Each oxidation oven heats the segments to a temperature approaching approximately 300° C. by means of a circulating flow of hot gas.
An example of such an oven is the Despatch Carbon Fiber Oxidation Oven, available from Despatch Industries, Minneapolis, Minn. A description of such an oven can be found in commonly-assigned U.S. Pat. No. 4,515,561. The oven described in the '561 Patent is a “center-to-ends” oxidation oven. In a center-to-ends oxidation oven, hot gas is supplied to the oxidation chamber of the oven from the center of the chamber and flows toward the ends of the chamber.
Typically, such a center-to-ends oxidation oven includes a center supply structure located in the center of the chamber. The center supply structure includes a plurality of supply plenums that are stacked one above each other. Gaps are provided between the plenums to enable passage of the fibrous segments between the plenums. Each plenum comprises a duct structure that receives heated air through one or both of its ends. Each plenum includes an array of holes formed in each of the opposing side walls of the corresponding duct structure. These holes are also referred to here as “nozzles”. Each plenum is configured to receive heated air and direct the flow of heated gas in approximately horizontal and parallel streams of heated gas out of the nozzles towards both ends of the oxidation chamber.
There is typically no uniform flow of gas in the gaps between the plenums. Whenever the line is stopped, the fibers are stationary. The portions of the fibers that happen to be positioned in the gaps between the plenums during such line stoppages can be subjected to temperatures that differ from the temperatures that exist within the remainder of the chamber due to the lack of uniform gas flow in the gaps between the plenums. As a result, those portions of the fiber may oxidize differently than the surrounding portions of the fibers.